Rincon de la Vieja

Costa Rica

10.83°N, 85.324°W; summit elev. 1916 m

All times are local (unless otherwise noted)

The active crater lake (150-200 m diameter) was gray to dirty white during May fieldwork, with weak, intermittent bubbling. Fumarolic activity in the E part of the crater, where water was slightly greenish, was stronger than during February fieldwork. The activity, audible at the crater rim, produced a plume that rose more than 100 m (the height of the crater wall), and was visible several kilometers N. Crater-lake level had dropped about 30 cm since February, while the temperature remained at 37°C and the pH at 1.6. Small mats of sulfur were visible on the lake surface. Weak vapor emission began at several points along a fissure (first observed in February) near the SE and SW rim, with temperatures of 55°C and 60°C, respectively.

Six microearthquakes were recorded in May (at OVSICORI station RIN3, 5 km S). A 16-minute tremor episode (1-2.5 Hz) was recorded on 22 May.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge that was constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of 1916-m-high Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A plinian eruption producing the 0.25 km3 Río Blanca tephra about 3500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.